The present invention relates to a cerium-based abrasive containing cerium oxide as a main component and excellent in polishing precision and cutting property as well.
Recently, a cerium-based abrasive containing cerium oxide (CeO2) has been used for polishing a variety of glass materials. Especially, a cerium-based abrasive has been employed for polishing from conventional common plate glass materials to glass materials today to be used for electric and electronic apparatuses, for example, glass for magnetic recording media such as hard disks, glass substrates of liquid crystal displays (LCD) and its application field is widened.
The cerium-based abrasive is composed of abrasive particles containing cerium oxide (CeO2) particles as a main component. The cerium-based abrasive can widely be classified into a high cerium abrasive and a low cerium abrasive depending on the content of cerium oxide. The high cerium abrasive contains at least 70% by weight of cerium oxide in relation to the whole amount of the rare earth oxides (hereinafter abbreviated as TREO) and is an abrasive containing a comparatively large amount of cerium oxide, whereas the low cerium abrasive is an abrasive containing cerium oxide in the content as relatively low as about 50% by weight in relation to TREO. Although the content of cerium oxide and the raw materials of these cerium-based abrasives are different, the production steps after raw material preparation are not so much different.
FIG. 1 shows the production steps of these cerium-based abrasives. The raw materials to be used for the high cerium abrasive are rare earth chlorides obtained by chemically treating and concentrating a rare earth ore so-called monazite. On the other hand, conventionally usually used as the raw materials of the low cerium abrasive is a bastnasite concentrate obtained by dressing the rare earth ore so-called bastnasite and recently, raw materials mainly used are derived from rare earth oxides or rare earth carbonates synthesized using the bastnasite ore and relatively economical complex ores produced in China. In the steps after raw material preparation, both are produced by chemically treating the raw materials (by the wet treatment), filtering, drying them, and further roasting them and pulverizing and sieving the obtained raw materials. The particle diameter of the abrasive particle constituting an abrasive is 0.2 to 3.0 xcexcm on the bases of the average particle diameter, although depending on the purposes from the rough finishing to the final finishing, and it is controlled by adjusting the temperature in the foregoing production steps and the roasting step and adjusting the pulverization and the classification steps.
By the way, one reason why a cerium-based abrasive has widely been employed is that a large quantity of glass can be removed by abrasion within a relatively short time to obtain a high polishing value in addition to that a polished face with high precision can be obtained. In this case, regarding the polishing mechanism of the cerium-based abrasive, there are not necessarily any clear established theories, however, it is said that the fluorine component contained in an abrasive takes a significant role. That is, in addition to the mechanical polishing effect by cerium oxide, it is said that also the chemical polishing function is simultaneously caused as follows: the fluorine component contained in the abrasive reacts with the glass face to fluorinate the glass and accelerate corrosion of the glass surface. Therefore, regarding the cerium-based abrasive, it is supposed to be possible to exert the excellent polishing properties when both of the mechanical function and the chemical function are sufficiently performed.
As the first standard to produce an abrasive with excellent polishing properties, first of all, it is required for the abrasive to be free from abnormally grown abrasive particles and to have evenness in the particle diameter distribution. Therefore, in the production steps of the cerium-based abrasive, a variety of countermeasures are performed so as to suppress the abnormal particle growth of the abrasive particles. For example, in the chemical treatment steps, a raw material pulverized with a mineral acid such as hydrochloric acid, sulfuric acid, and the like is treated. That is for dissolving and removing alkali metals, e.g. sodium, and alkaline earth metals contained as impurities in the raw material with mineral acids since these alkali metals and alkaline earth metals cause the abnormal particle growth in the roasting step thereafter.
Further, in addition to the control in these production steps, regarding produced abrasives, product inspection is randomly performed to investigate the average particle diameter and the particle diameter distribution to investigate the existence of coarse particles owing to the abnormal particle growth.
Excellent abrasives have been supplied ever before through the above described control of the particle diameter distribution of abrasive particles in the production steps and the fluorine concentration and product inspection.
However, taking the demands for the cerium-based abrasive in the future into consideration, it is natural to be desired to develop further excellent abrasives. Especially, since further integration to a high density is required in an industrial field of a glass substrate for hard disks, LCD and the like and for that, it is required to obtain an abrasive which not only is capable of forming a polished face with extremely high precision but also has a high cutting property to carry out polishing to a prescribed extent at a high speed.
Hence, the present invention aims to provide a cerium-based abrasive capable of forming a polished face with precision higher than that a conventional method has ever achieved and having an excellent cutting property and a new standard and to provide a production method.
The inventors of the present invention have made investigation in details in polishing properties of conventional cerium-based abrasives and found a problem that scratches are sometimes formed in the polished face even in the case of using a conventional abrasive with a finely miniaturized average particle size and of which the particle diameter distribution is found no problem in a product inspection. Then, a variety of investigations into the causes of the occurrence of such scratches have been performed and consequently, the following two problems are found in the conventional abrasives which are regarded not to have any problem in the particle diameter distribution by the above described inspection method and thus the present invention is achieved.
The first problem in the conventional cerium-based abrasives is the existence of coarse particles in an extremely slight amount, which are inevitably contained by a conventional production method and which are impossible to be detected by a conventional inspection method. In this case, the coarse particles- in the present invention mean particles having a particle diameter several times as large as the average particle diameter of the abrasive particles composing an abrasive and having a particle diameter within a range in which they can not be detected by the particle diameter distribution by a conventional inspection method and within a range of 10 to 50 xcexcm or more.
The inventors of the present invention have thought it is necessary to clarify the relation between the coarse particle concentration and the polished face even if the amount is extremely slight in order to obtain an abrasive with higher precision ever before. From such a viewpoint, the inventors of the present invention have enthusiastically investigated and consequently found that even if those found having no problem by a conventional inspection method contain coarse particles with a diameter of 10 xcexcm or larger in 1,500 ppm or more by weight.
The inventors of the present invention have then found as a first invention that an abrasive can be provided corresponding to the applications by optimizing the coarse particle concentration according to the results of production of a variety of cerium-based abrasives with different coarse particle concentrations by production methods performed by variously modifying a conventional production method as described somewhere later and investigation of the frequency of the occurrence of the scratches in the polished face.
In other words, the first invention of the present application is a cerium oxide-based abrasive containing cerium oxide with an average particle diameter of 0.2 to 3.0 xcexcm as a main component, wherein the cerium oxide-based abrasive contains coarse particles with a particle diameter of 10 xcexcm or larger in a concentration of 1,000 ppm or lower by weight.
The cerium-based abrasive according to the first invention is one containing the coarse particles in a lowered concentration as compared with that of a conventional one and such an abrasive having 1,000 ppm or lower by weight concentration of the coarse particles of 10 xcexcm or larger is suitable for polishing the glass face of a Braun tube and a CRT. By using such an abrasive, polishing the glass face of these products can provide a polished face with desired precision without forming scratches.
Further, according to the findings of the inventors of the present invention, it is confirmed that as the concentration of the coarse particles is lowered, the scratch formation can be suppressed more to obtain a polished face with desired precision. Especially, regarding the requirement for a polished face with extremely high precision for the finishing polishing of a glass substrate or the like for a hard disk and a LCD, today, a polished face scarcely having scratches has been required. It is therefore preferable to use an abrasive containing coarse particles with 10 xcexcm or larger particle size in the concentration of 300 ppm by weight or lower in polishing at such high precision.
The second problem of a conventional cerium-based abrasive is the existence of the magnetic particles composed of magnetic metals of iron, nickel, and the like. Such magnetic particles are supposed to be contained in raw materials from the beginning or mixed during the production steps. Especially, in the production steps, contamination takes place supposedly attributable to the constituent materials of a wet pulverize, a drier, a roasting apparatus, a dry pulverize, and the like in the steps such as a wet pulverization step, a drying step, a roasting step, a dry pulverization step. The reason why the magnetic particles become causes of the occurrence of the scratches is supposed to be that the difference of the hardness between the magnetic particles (metal particles) and the abrasive (a metal oxide of such as cerium oxide) makes uniform polishing impossible.
The inventors of the present invention have made investigations of the relation between the content of the magnetic particles and the frequency of scratch occurrence based on the above described findings and have thought it is possible to obtain a highly precise cerium-based abrasive with no probability of causing scratches by defining the concentration of the magnetic particles within a prescribed range and achieved the second invention of the present application.
The second invention of the present application is a cerium oxide-based abrasive containing cerium oxide with an average particle diameter of 0.2 to 3.0 xcexcm as a main component, wherein the cerium oxide-based abrasive contains magnetic particles in a concentration of 1,000 ppm or lower (by weight).
The cerium-based abrasive according to the second invention is wherein the content of the magnetic particles of a conventional cerium-based abrasive is lowered to suppress the concentration of the magnetic particles to 1,000 ppm or lower. The reason why the concentration of the magnetic particles is suppressed to 1,000 ppm or lower is because the concentration of the magnetic particles of a conventional cerium-based abrasive causing the scratches is found to be 1,500 ppm or higher by weight and the restriction is based on the consequent finding by investigations that the occurrence of scratches can reliably be suppressed to 1,000 ppm or lower.
The inventors of the present invention have confirmed that as the concentration of magnetic particles is lowered, the occurrence of the scratches is suppressed more to obtain the polished face with high precision. The abrasive containing magnetic particles in a concentration of 1,000 ppm or lower by weight is suitable, for example, for polishing glass face of a Braun tube and a CRT. On the other hand, regarding finishing polishing of a glass substrate or the like for a hard disk and a LCD, since extremely high precision of the polished face has today been required, the requirement can be satisfied by further decreasing the concentration of the magnetic particles and it is preferable to use those containing magnetic particles in a concentration of 300 ppm by weight or lower. Further, especially for a hard disk, if magnetic particles remain on the substrate surface after polishing, it causes a significant problem in magnetic recording and reproduction systems to result in deterioration of the reliability of the hard disk. Consequently, by using such an abrasive with a decreased concentration of magnetic particles of the present invention, the quality of a hard disk produced can be improved.
As described above, cerium-based abrasives according to the present invention contain coarse particles and magnetic particles in contents within respectively prescribed ranges, and these cerium-based abrasive can be said to be highly precise abrasives with no probability of causing scratches on the polished face.
Further, the inventors of the present invention have found a cerium-based abrasive having an excellent cutting property and capable of forming a polished face with high precision not only by controlling the concentration of the coarse particles and the concentration of the magnetic particles but also by controlling the specific surface area to within a range of 0.5 to 30 m2/g.
If the specific surface area is smaller than 0.5 m2/g, the abrasive particles become so large owing to excess proceeding of sintering that it is supposed scratches are caused at the time of polishing. Further, if the specific surface area is larger than 30 m2/g, the abrasive particles become so small owing to the insufficient degree of sintering that it is supposed the cutting property is too lowered.
As described above, a cerium-based abrasive comprising abrasive particles with a specific surface area within a prescribed range and having a decreased concentration of coarse particles and a decreased concentration of magnetic particles is capable of forming the polished face with high precision and taking an expected vast expansion of applications of a cerium-based abrasive in the future into consideration, it is necessary to stably produce a large quantity of the abrasive. The inventors of the present invention have reviewed the conventional production steps of a cerium-based abrasive and found that the concentration of coarse particles, the concentration of magnetic particles, and the specific surface area of the abrasive particles can easily be controlled by altering the production conditions. In this case, the conventional steps for producing a cerium-based abrasive can widely be divided into a step of forming a slurry by mixing an abrasive raw material and a dispersing medium, a step of pulverizing the abrasive raw material by treating the slurry by a wet pulverize, a step of roasting the abrasive raw material subjected to the pulverization after filtration and drying, and a step of again classifying the abrasive raw material after it is subjected to the roasting followed by repulverization.
A method for lowering the concentration of coarse particles, which is the first characteristic of the present invention, comprises the following two improvements in the classifying step.
The first improvement is to lower the concentration of the coarse particles by setting the classifying point to 0.5 to 15 xcexcm. In this case, the classifying point denotes the particle diameter to be a boundary of particles to be separated by a classifying apparatus and it generally means the particle diameter value at which the partial classification efficiency (the particle amount in the separated fine particle side to the particle amount before the classification) becomes 50%. The classifying point is generally corresponding to the operation conditions (e.g. the air delivery, the rotation speed, and the like in the case of a wind type classifying apparatus) and in the present invention, the classification point is fixed to carry out the classification treatment by setting the operation conditions of the classifying apparatus. The reason why the range of the classification point is set from 0.5 to 15 xcexcm is that in the case of 15 xcexcm or larger, the concentration of coarse particles cannot be lowered and on the other hand that even in the case of setting the classification point to be 0.5 xcexcm or smaller, it becomes difficult to set the operation conditions of the classifying apparatus corresponding to the setting. Further, taking the production efficiency of an abrasive into consideration, the classification point especially preferable is within a range from 1 to 10 xcexcm.
Incidentally, the classification treatment involving such alteration of classification point does not restrict the models of classifying apparatuses. Consequently, the treatment is made possible by any types of apparatuses, e.g. so-called a dry classifying apparatus such as a wind type classifying apparatus and so-called a wet type apparatus such as a liquid cyclone.
The second method for lowering the concentration of coarse particles is that re-classification of an abrasive after classification is carried out at least once. That is based on the consideration that there is possibility of occurrence of contamination of coarse particles with the size equal to or larger than the classification point in the abrasive by only once classification even if the classification point is limited. By carrying out re-classification, the concentration of the coarse particles in the abrasive can be lowered. The number of times to repeat the re-classification is sufficient to be about once taking the production efficiency of the abrasive into consideration. Incidentally, the re-classification is preferable to be carried out in combination with the above described control of classification point, that is, the classification point is restricted within a range of 0.5 to 15 xcexcm. Consequently, it is made easy to produce an abrasive having the concentration of coarse particles in 300 ppm or lower and possible to be used for finishing polishing. Incidentally, the classification point at the time of re-classification in that case may be set at the same value of the classification point at the time-of first classification.
On the other hand, as a method for lowering the concentration of magnetic particles, the following three are available. A first technique to lower the magnetic particles of an abrasive comprises a step of passing a slurry treated by a wet type pulverize through a magnetic material magnetized by excitation. The reason for the removal of magnetic particles from an abrasive raw material in the slurry state is because if being in the slurry state, the particles are scarcely agglomerated and the magnetic particles are made easy to move by the magnetic field, so that the magnetic particles can efficiently be collected more practically, magnets are arranged in the circumference of a filter and the magnets are excited to apply a magnetic field to the filter part to magnetize the filter and then a slurry of the abrasive material is passed through the filter. In this case, electromagnets are preferable to be employed as the magnets to magnetize the magnetic filter and the intensity of the magnetic field to be generated is preferable to be within a range from 0.1 to 2.0 T magnetic flux density. In the case of 0.1 T or lower, small magnetic particles cannot be collected, whereas in the case of 2.0 T or higher, the operation cost of the magnets is increased and additionally it becomes difficult to remove the attracted magnetic particles adhering to the filter at the time of maintenance to result in decrease of the productivity. The number of the times to repeat the filtration of the abrasive slurry is not particularly restricted, however by setting the number to be a plurality of times, the concentration of the magnetic particles can further be decreased.
Incidentally, the models of a wet type pulverize to be employed in this case are not particularly restricted, however, applicable are, for example, a wet type ball mill, a bead mill, an attriter and the like. Further, at the time of carrying out wet pulverization of the abrasive raw material, it is necessary to make the abrasive raw material be a slurry by mixing it with a dispersing medium, and procedure at the time includes a case carrying out the steps of previously mixing the abrasive raw material with a dispersing medium to produce a slurry and then pulverized by loading a wet type pulverize with the slurry and besides the case, also includes a case of carrying out the steps of loading a wet type pulverize with an abrasive raw material and a dispersing medium to start pulverization and make the abrasive raw material be a slurry at the initial period of the pulverization. These cases are similarly applicable for the following production methods.
As a second technique to lower the magnetic particles of an abrasive includes steps of pulverizing the abrasive raw material using a pulverization medium made of a non-magnetic material with which a wet type pulverize is filled to pulverize a slurry to be pulverized. The pulverization medium is an object which moves interlockingly with the movement (rotation, vibration) of the pulverize while being packed in the pulverize and consequently performs pulverization function and ball-like ones are used for the above described wet type ball mill or the like. The material to be used for the medium is generally iron or a wear resistant steel in consideration of the cost and the pulverization efficiency.
In this case, as described above that the cause of the contamination of the abrasive with magnetic particles is attributed to the raw material pulverization step among the production steps, that is supposedly attributed to that wear or pulverization of the pulverization medium is caused during the pulverization step and it becomes one of causes of contamination with magnetic particles. The second method aims to inhibit generation of magnetic particles by using a non-magnetic material for the material of the pulverization medium and consequently to lower the concentration of magnetic particles in the abrasive. Hence, taking the intrinsic function of pulverizing a raw material into consideration, it is preferable to use a non-magnetic and hard material such as zirconia or alumina for the constituent material of the pulverization medium. Incidentally, in the case where a non-magnetic material for the pulverization medium, it is preferable to use a hard non-magnetic material for the inner parts of a pulverizer and the inner faces of pipes and tubes to be brought into contact with the slurry in the pulverize.
As a third technique to decrease the magnetic particles in an abrasive, an abrasive raw material after roasting is passed through the peripheral part of a tubular body, a plate-like body, and a rod-like body made of a magnetized magnetic material. Among the abrasive production steps, the step is the treatment for the abrasive material in form of almost the final product shape in terms of the composition after the roasting and pulverization steps and before classifying step and the dry treatment in contrast with the treatment for the slurry state. The third method is effective to remove mixed magnetic particles derived from the constituent materials of the roasting furnace or the pulverize in the roasting step or the pulverization step after the roasting. Incidentally, the dry type magnetic particle removal step is a treatment for the abrasive raw material after the roasting and may be carried out after dry type pulverization immediately after the roasting or before the classification treatment and may be carried out at any timing after the dry pulverization or after the classification treatment. Further, a practical embodiment of the method includes that the abrasive raw material after the roasting is passed through the inside of a tubular body or a box body magnetized by excitation, that the abrasive raw material is brought into contact with magnets arranged in a transportation passage for the abrasive raw material after the roasting, and that the abrasive raw material is passed between two magnetized metal plates or metal rods. Further, the magnetic flux density of a magnetic material to be magnetized is preferably within a range from 0.1 to 2.0 T according to the same reason as already described in the case of the magnetic filter.
By the above described three methods for decreasing the magnetic particle concentration, magnetic particles can efficiently be removed without requiring additional installation of large scale facilities and a cerium-based abrasive with a high quality can be produced at a low cost. Although the magnetic particle removal treatments are of course effective even if the respective treatments are carried out separately, the concentration of magnetic particles can further efficiently be decreased by combining two or all three methods.
Hence, regarding a cerium-based abrasive as described above, a cerium-based abrasive capable of forming a polished face with a precision and having excellent cutting property can be obtained not only by controlling the concentration of coarse particles and the concentration of magnetic particles but also by controlling the specific surface area to be within a range from 0.5 to 30 m2/g. As described above, in order to keep the specific surface area of an abrasive within the above described range, it is important to control the sintering conditions. Although depending on the sintering method and the particle diameter of the abrasive raw material before the sintering, the sintering conditions are preferably controlled to be 600 to 1100xc2x0 C. of sintering temperature and 1 to 60 hours of sintering duration.
Incidentally, in the above described production method of the cerium-based abrasive, it is required to disperse a raw material in a dispersion medium at the time of wet pulverization of the raw material, an organic solvent may be usable for the dispersing medium, however those containing mainly water are preferable. Further, the dispersion medium is preferable to be properly mixed with a dispersant, a pH adjusting agent and the like.
Further, in a cerium-based abrasive of the present invention with decreased coarse particles or magnetic particles, fluorine is properly added at the time of production if necessary to carry out chemical treatment.